The present invention relates to spray cast copper alloys having reduced porosity. More particularly, the alloy is combined with an addition capable of dissolving the atomization gas.
The formation of coherent metallic articles by spray casting is disclosed in U.S. Pat. Nos. RE 31,767 to Brooks and 4,804,034 to Leatham et al. both of which are incorporated herein by reference in their entireties.
Spray casting involves passing a molten metal stream through a gas atomizer in a protective atmosphere. The atomizer is supplied with a pressurized gas which atomizes the metal and provides a protective atmosphere to prevent oxidation of the droplets. The droplets are broadcast downward from the atomizer in the form of a divergent conical pattern and strike a collector. The collector is spaced sufficiently close to the atomizer so that the droplets striking the collector are in a partially solidified state. Solidification is completed following impact with the collector. The atomized droplets coalesce on impact forming a coherent structure.
One objective of spray casting is to achieve a fully dense deposit. Frequently, the density of the spray cast deposit is only about 95% of the theoretical value. One reason for the low density is porosity.
The causes of porosity in spray casting are not fully understood. One probable mechanism is that when the cast droplets accumulate on the collector, nitrogen bubbles can become trapped at the interstitial voids between droplets. The nitrogen, which is insoluble in copper, causes bubbles which prevent the flow of metal into the interstitial voids resulting in the formation of pores when the metal solidifies completely.
Pores cause a myriad of problems. In addition to reducing the density of the casting, the pores form crack initiation sites which can cause a fracture. The pores will also reduce the electrical conductivity of the spray casting.
One method of reducing porosity is disclosed in U.S. Pat. No. 5,017,250 to Ashok et al which is assigned to a common assignee and incorporated herein by reference. Ashok et al disclose adding an element to the molten stream which reacts with the atomization gas. For example, zirconium, titanium, aluminum or chromium are used when nitrogen is the atomization gas. In conformance with the above theory of porosity, the reactive elements react with nitrogen at the interstitial voids forming a solid nitride dispersoid.
Applicants have developed a different means for reducing the porosity of a spray cast copper alloy which does not result in the formation of a second phase dispersoid. Having determined that nitrogen is soluble in certain copper alloys containing manganese, sufficient manganese is added to the alloy so that nitrogen which collects at the interstitial voids is dissolved into the alloy resulting in improved coalescence of the droplets and a more dense casting.